AFE5812_15 Datasheet, PDF (82/109 Pages) Texas Instruments – AFE5812 Fully Integrated, 8-Channel Ultrasound Analog Front End with Passive CW Mixer, and Digital I/Q Demodulator, 0.75 nV/rtHz, 14/12-Bit, 65 MSPS, 180 mW/CH

AFE5812
SLOS816A – MARCH 2015 – REVISED MARCH 2015
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Typical Application (continued)
11.2.1 Design Requirement
The AFE5812 is a highly integrated analog front-end solution. In order to maximize its performance, users must
carefully optimize its surrounding circuits, such as T/R switch, Vcntl circuits, audio ADCs for CW path, clock
distribution network, synchronized power supplies and digital processors. Some common practices are described
below.
Typical requirements for a traditional medical ultrasound imaging system are shown in Table 27.
Table 27. Design Parameters
PARAMETER
Signal center frequency (f₀)
Signal Bandwidth (BW)
Overloaded signals due to T/R
switch leakage
Maximum input signal amplitude
Transducer noise level
Dynamic range
Time gain compensation range
Total harmonic distortion
EXAMPLE VALUES
1~20 MHz
10~100% of f₀
~2 Vpp
100 mVpp to 1 Vpp
1 nV/rtHz
151 dBc/Hz
40 dB
40 dBc @ 5MHz
11.2.2 Detailed Design Procedure
Medical ultrasound imaging is a widely-used diagnostic technique that enables visualization of internal organs,
their size, structure, and blood flow estimation. An ultrasound system uses a focal imaging technique that
involves time shifting, scaling, and intelligently summing the echo energy using an array of transducers to
achieve high imaging performance. The concept of focal imaging provides the ability to focus on a single point in
the scan region. By subsequently focusing at different points, an image is assembled.
When initiating an imaging, a pulse is generated and transmitted from each of the 64 transducer elements. The
pulse, now in the form of mechanical energy, propagates through the body as sound waves, typically in the
frequency range of 1MHz to 15 MHz. The sound waves are attenuated as they travel through the objects being
imaged, and the attenuation coefficients ɑ are about 0.54 dB/(MHz×cm) in soft tissue and 6~10 dB/(MHz×cm) in
bone ( source: http://en.wikipedia.org/wiki/Attenuation). Most medical ultrasound systems use the reflection
imaging mode and the total signal attenuation is calculated by 2×depth×ɑ×f₀. As the signal travels, portions of the
wave front energy are reflected. Signals that are reflected immediately after transmission are very strong
because they are from reflections close to the surface; reflections that occur long after the transmit pulse are
very weak because they are reflecting from deep in the body. As a result of the limitations on the amount of
energy that can be put into the imaging object, the industry developed extremely sensitive receive electronics
with wide dynamic range.
Receive echoes from focal points close to the surface require little, if any, amplification. This region is referred to
as the near field. However, receive echoes from focal points deep in the body are extremely weak and must be
amplified by a factor of 100 or more. This region is referred to as the far field. In the high-gain (far field) mode,
the limit of performance is the sum of all noise sources in the receive chain. In high-gain (far field) mode, system
performance is defined by its overall noise level, which is limited by the noise level of the transducer assembly
and the receive low-noise amplifier (LNA). However in the low-gain (near field) mode, system performance is
defined by the maximum amplitude of the input signal that the system can handle. The ratio between noise levels
in high-gain mode and the signal amplitude level in low-gain mode is defined as the dynamic range of the
system. The high integration and high dynamic range of the device make it ideally suited for ultrasound imaging
applications.
The device includes an integrated LNA and VCAT (which use the gain that can be changed with enough time to
handle both near- and far-field systems), a low-pass antialiasing filter to limit the noise bandwidth, an ADC with
high SNR performance, and a CW mixer. Figure 98 illustrates an application circuit of the device.
Use the following steps to design medical ultrasound imaging systems:
1. Use the signal center frequency and signal bandwidth to select an appropriate ADC sampling frequency.
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